Bacterial conjugation is a process whereby a conjugative plasmid is transferred from a donor to a recipient. The circular plasmid is transferred as single-stranded DNA;therefore one plasmid strand must be cleaved in the donor and ligated in the recipient. For conjugative plasmid F, TraI is the central player in the cleavage and ligation processes. TraI, a relaxase or nickase, cleaves single-stranded plasmid DNA with remarkable sequence specificity. DNA nicking, which causes formation of a stable linkage between TraI and the DNA strand, occurs while TraI participates in a multi-protein complex called the relaxosome. In addition to its relaxase activity, TraI possesses a helicase activity. Efficient transfer requires that relaxase and helicase activities be contained within the same protein, and a switch between these activities may be an important regulatory step in F conjugative transfer. In one model for F transfer, TraI cleaves plasmid DNA as part of the relaxosome, dissociates upon receiving a signal initiating transfer, pilots the leading end of the DNA out of the donor and into the recipient, tracks along the incoming DNA using its helicase activity, and ligates the plasmid ends together to conclude transfer. Using a combination of genetic, biochemical, single molecule fluorescence and structural techniques, we propose experiments designed to answer several key questions about TraI and conjugation initiation: Can we observe individual relaxosomes in a cell in real time? If so, where is the relaxosome located within the donor cell relative to the conjugative pore through which the DNA is transported? Does the relaxosome location change when donors and recipients interact? What characteristics of TraI are required to form the relaxosome? Is TraI transferred to the recipient during transfer, and if so can we detect TraI in the recipient? If TraI is transferred, is it transferred in a folded or denatured state? If denatured, does the VirB4 homologue TraC act as an unfoldase? Does the conformation of TraI change when it interacts with its relaxase and its helicase DNA substrates? Could such a conformational change or could negative cooperativity of binding of DNA to the relaxase and helicase regions of TraI explain the conversion between roles of TraI? The research will not only provide insight into the molecular mechanisms required for conjugative transfer, but they will also contribute to our general understanding of the regulatory mechanisms of large multifunctional proteins. Bacterial conjugation facilitates genetic exchange between bacteria, assisting genome diversification and evolution of enteric pathogens. Conjugative plasmids also can contribute to bacterial pathogenesis via biofilm formation and other mechanisms, and are potential tools for facilitating gene replacement during gene therapy. The proposed experiments will provide a better understanding of the mechanism of conjugation, eventually allowing manipulations designed to improve human health.